Updated · 9 min read
In short
- The U-value measures heat loss through 1 m² of wall per degree of temperature difference — lower is better.
- Insulation performance depends on thickness ÷ thermal conductivity (λ); materials with a lower λ insulate better at the same thickness.
- The first centimetres bring by far the largest improvement; moisture behaviour and thermal bridges matter as much as the thickness.
Why walls lose heat
Heat always flows from warm to cold. In winter, an uninsulated solid wall conducts heat from the room to the outside air at a rate set by the wall's materials and thickness. Insulation is a layer with very low thermal conductivity that slows that flow down.
Two numbers describe it. The thermal conductivity λ (lambda), in W/(m·K), is a property of the material: the lower, the better it insulates. The thermal resistance R of a layer is its thickness in metres divided by λ. Add up the resistances of all layers plus two small surface resistances, take the inverse, and you have the U-value.
What the U-value tells you
The U-value, in W/(m²·K), is the heat flow through one square metre of wall for each degree of difference between inside and outside. A wall with U = 1.0 loses 20 watts per square metre when it is 20 °C inside and 0 °C outside; a wall with U = 0.2 loses only 4 watts.
Uninsulated solid masonry walls are often between 1.5 and 2.5 W/(m²·K). Insulated walls in modern energy-efficient buildings are typically around 0.15–0.30. Which value is required for your building depends on national regulations and on whether it's a new build or a renovation.
How the common materials compare
The ranges are typical published values; always use the declared value from the product you buy. Fire safety rules, especially on taller buildings, may dictate which materials can be used where.
| Material | Typical λ, W/(m·K) | Notes |
|---|---|---|
| EPS (white) | 0.035–0.040 | Light, inexpensive, common for external wall insulation; combustible |
| EPS (graphite, grey) | 0.031–0.032 | Same thickness insulates better; protect from sun during installation |
| Mineral wool (stone or glass) | 0.032–0.040 | Non-combustible, vapour-open, good sound insulation |
| XPS | 0.033–0.038 | Closed-cell, takes little water: plinths, below ground, under slabs |
| Wood fibre | 0.038–0.050 | Vapour-open, good summer heat protection, heavier |
| PIR / PUR boards | 0.022–0.028 | Very thin for the performance; higher price |
How much thickness pays off
Because U is the inverse of the total resistance, every extra centimetre of insulation improves it by less than the previous one. For a 25 cm solid brick wall insulated with EPS (λ = 0.035):
| Insulation thickness | U-value, W/(m²·K) | Heat loss vs. uninsulated |
|---|---|---|
| 0 cm | 2.09 | 100% |
| 5 cm | 0.52 | 25% |
| 10 cm | 0.30 | 14% |
| 15 cm | 0.21 | 10% |
| 20 cm | 0.16 | 8% |
| 30 cm | 0.11 | 5% |
External, internal or cavity insulation
- External insulation (an insulated render system or a ventilated façade) is the most effective: it covers the whole wall, including floor edges and many thermal bridges, and keeps the masonry warm and dry.
- Internal insulation is used when the façade can't be changed, for example on listed buildings. It takes floor space and moves the cold zone inside the wall, so the risk of condensation must be checked; vapour-open systems or a carefully installed vapour control layer are needed.
- Cavity wall insulation fills an existing air gap between two masonry leaves, where the construction allows it.
Moisture and thermal bridges
A well-insulated wall can still develop mould where heat escapes locally: window reveals, balcony slabs, the top of the wall below the roof, the plinth. Plan these details together with the insulation, not afterwards.
Insulating also changes how moisture moves through the wall. For internal insulation and for unusual constructions, ask for a hygrothermal assessment from an energy adviser or building physicist.
Before you start
- Have the existing wall assessed: material, thickness, moisture, cracks.
- Check the local rules for energy performance, fire safety and planning — they differ by country and sometimes by municipality.
- If you apply for a grant, find out the technical requirements first; they usually have to be met and documented by a qualified adviser.
Frequently asked questions
Is 10 cm of insulation enough?
On a solid brick wall, 10 cm of EPS already cuts heat loss through the wall by about 86%. Whether it meets your target depends on the regulation or the energy standard you are aiming for — use the U-value calculator to compare thicknesses.
Does insulation make a house damp?
Correctly designed external insulation usually makes walls warmer and drier. Problems arise from poorly detailed internal insulation, gaps in vapour control layers and thermal bridges — plan the details.